Color is linked when duplicating object
When you duplicate an object, by default the copy keeps the same material as the original. If you want to create a new material for this copy you need to:


2.79: Press U (Make Single User) > Object &amp; Data &amp; Material
2.8: Header menu > Object > Relations > Make Single User > Object &amp; Data &amp; Material.


In the case of a linked duplication (AltD), go in the Properties panel > Material > On the right of the material name click on the dropdown menu and choose Material, this way the meshes of the objects are still linked but not their material.